Brake became solid when driving

Hi ,I wondered if anyone has ever experienced this problem ,it’s on an xk150 , i took the car and thought the brake pedal was a bit hard ,after about two miles the car became slower and slower ,in the end i just managed to get to a layby ,the engine was by now very hot and the smell of burning brakes was terrible ,i called out a breakdown company and sat waiting , after two hours ,i thought I’d try to move the ,it drove ok for half a mile and started locking again ,and i purposely didn’t use the brakes ,i got it back towards home and it completely locked on again the couldn’t move the car , i sat in it again and suddenly the brake pedal shot up on its own and the car was free moving again ! I got it home this time ,but there’s obviously something wrong ,has anyone any ideas .Chris Hedley , sorry its long winded !

Try loosening the bake actuator rod.

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Sticking air/vacuum piston in the brake servo? Try pouring a few drops of brake fluid down the breather pipe.

Or

Collapsing brake hoses?

The pedal pivots on a cage roller bearing…if not maintained/greased they can get rusted up…also check pedal return spring…and master cylinder piston could be sticking…so you have a few things to look at…Steve

Pat, I agree-I have had this problem on my E Type, my TR3 and even my 62 Chrysler. I think iy is surprising that warm fluid will expand enough that it begins to engage the brakes which further heats the fluid etc etc. Good luck! Mike Moore

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Thanks for all the replies and suggestions ,I’m away on holiday for few days ,will get at it when im back ,and let you know what is found .

Thanks again Chris Hedley

The pressure is not releasing. This exact symptom happened on my XK120. Brakes gradually locking up, 3 men could not push the car an inch. Released after sitting about an hour. Silicone brake fluid caused the rubber seals inside the master cylinder to swell in the axial direction, and it was just enough for the seal to block the tiny pressure release port where the fluid normally passes between the reservoir and the main piston chamber. With the master on the workbench I could stick a pin down the passage and feel the rubber seal, where it should not have been.
I refer to port X in this picture. Seal F became too long and blocked it.

It also happened on my XJ12 (disc brakes) where the silicone fluid caused the seals in the caliper pistons to swell up and the pistons became so stuck that they would not move, i.e. would not release pressure. When I removed the seals, they were not cylindrical, more in the shape of a truncated cone. I soaked them in a jar of Castrol LMA for a week and they returned to cylindrical shape.

Silicone has been banned from my garage forever.

In my case, precisely the same, was solved by adjusting the actuating rod. As in Rob’s case, the pressure relief was the culprit, but in my case not due to seals but to poor adjustment that did not let the piston travel back enough.

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I agree with the previous diagnoses above that you do not have sufficient freeplay of the M/C pushrod, though your description of the pedal “shooting up” doesn’t fit - that’s something you need to confirm that’s actually happening, or does it seem like it’s raising back up because you can actuate it again after letting it sit awhile after it’s become rock hard? When troubleshooting what sounds like the same issue in my 120 about a month ago I got under the car and pulled back the rubber boot on the master to inspect the end of the pushrod while I actuated the pedal and discovered that the generous freeplay I thought I had was entirely in the cylinder - the end of the pushrod was touching the primary piston, which was sliding forward in the cylinder before any resistance was felt in the pedal. Adjusting the rod backward to result in a 1/32” gap cured the problem.

Brake fluid expands with heat, and braking generates a lot of it. Normally when you let off on the brakes the piston moves back, the seal in the master is broken and the expanded brake fluid is allowed to flow back upwards into the reservoir, which is at atmospheric pressure. But this happens only if the piston moves fully back. If the piston can’t move back fully the high pressure in the cylinder isn’t released and the brakes lock up until such time as the fluid cools sufficiently to reduce its volume.

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Hi Nick , yes the brake pedal did suddenly come up on it’s own , as I said i drove the car back making sure i didn’t touch the brake , all in all about a mile , and it was the last time it had jammed on and i pulled over that, after being locked on that it did come up with a bang, this of course very annoying as it worked ok after ,I will look at the master cylinder when I get back from holiday .Thanks to everyone for the input , regards Chris

It may be an internally collapsed rubber hose too, they should be routinely replaced every 20yrs or so

Did you resolve your issue, Chris?

Hi nick ,i have just got back from holiday ,I had a look and its definitely something on the brake master cylinder ,im going to book it in at a jaguar specialist near where I live ,i will reply when it is sorted .Thanks for your reply ,appreciated .Chris